From Sahih Muslim (Hadith 4656):
It has been narrated on the authority of Aswad b. Qais who heard Jundub saying that Gabriel delayed his visit to the Messenger of Allah (ﷺ) The polytheists began to say that Muhammad has been forsaken. At this Allah, the Glorious and Exalted, revealed:" Wa'dd hd wa'l-laili iza saja, ma wadda'ka Rabbuka wa' ma qala" [By the glorious morning light, and by the night when it is still: thy Lord has not forsaken thee, nor is He displeased]
Reference: Muslim 4656